1. Field of the Invention
The present invention relates to optical communication systems. In particular, the present invention provides a novel configuration of optical communication systems comprising an optical transmission line consisting of optical fibers, and optical fiber amplifiers.
2. Description of Related Art
Optical communication systems using an optical fiber as a signal transmission line have been developed. These systems benefit from the low loss and wide band properties of the optical fiber. Therefore, with regard to the use of optical fibers in optical communication systems, various technologies and applications such as coherent light transmission systems, intensity-modulation/direct-detection systems, etc., have been studied.
One technology recently being studied is an optical fiber amplifier which uses an amplification effect of an Er-added (Erbium added) optical fiber or the like. An Er-added optical fiber is fabricated by doping a conventional optical fiber material with the rare earth element of Erbium. Such an optical fiber amplifier has a number of features such as high gain, low insertion loss, no polarization dependence, low noise, and high saturation output, etc. A successful non-repeating transmission experiment conducted on this type of amplifier resulted in a bit rate of 1.8Gb/sec for a distance of 212 km (The Institute of Electronics, Information and Communication Engineers of Japan, Light Communication System Study Society OCS 89-3 (K. Hagimoto, et al., PD 15, Optical Fiber Communication Conference '89)). In addition, using the amplifier in cooperation with an optical transmission line in a system having multiple independent receivers (i.e. cable TV subscribers) has been considered (The Institute of Television Engineering of Japan, Technical Report 1989 (K. Kikushima, et al., PD 22, Optical Fiber communication Conference '90)). In general, the development of optical fiber amplifiers is expected to contribute greatly to the improvement of future optical communication systems.
FIG. 4 (Prior Art) is a schematic diagram of a basic configuration of an optical fiber amplifier using an Er-added optical fiber.
The optical fiber amplifier 210 (surrounded by the dotted line in FIG. 4) comprises an optical fiber 31 having an input terminal connected to an optical transmitter 200, an optical fiber 32 having an output terminal connected to an optical receiver 202, and an Er-added optical fiber 35 coupled between the optical fibers 31 and 32 through optical isolators 33 and 34. An pumping light source 36 is coupled with the Er-added optical fiber 35 through a multiplexing photo-coupler 36a. In order to eliminate pumping light and ASE (Amplified Spontaneous Emission) from an optical transmission line, a filter 35a is inserted between the optical isolator 34 and optical fiber 32.
Optical isolators 33 and 34 are non-reciprocal optical elements which transmit light in one direction only. The optical isolators 33 and 34 suppress the laser oscillation of the Er-added optical fiber 35. Therefore, in this system, all light signals are prevented from propagating to the transmitter from the receiver by the optical isolator 33 or 34.
However, in a practical operation of the optical communication system, it may be necessary to perform bi-directional optical signal transmission through a single optical fiber line, or to monitor the conditions of an optical transmission line by use of an optical time domain reflectometer (OTDR). By monitoring Rayleigh back scattering light produced in the optical fiber at one terminal portion of the optical fiber, the OTDR can detect various defective conditions (i.e. disconnections) which may occur at any portion of the optical fiber.
FIG. 5 (Prior Art) is a schematic diagram of an example of an optical communication system using an optical fiber amplifier comprising an Er-added fiber. An optical signal is transmitted from a center office 400 via an optical fiber amplifier 410 to a subscriber 420.
An OTDR 41 and an optical transmitter 42 are provided in the center office 400. The OTDR 41 and the optical transmitter 42 are connected to an optical fiber 44 through a common multiplexing photo-coupler 43. The optical fiber 44 acts as a transmission line and is connected to an Er-added optical fiber 46 through an optical isolator 45, and the end terminal of the Er-added optical fiber 46 is connected to an input port of a star coupler 48 through an optical isolator 47. An output port of the star coupler 48 is connected to a plurality of optical fibers 49a which are each connected to a receiver 49. In practice, the optical isolator 45 is connected to the Er-added optical fiber 46 through a multiplexing photo-coupler 46a which is adapted to inject pumping laser light provided by a semiconductor laser 46b to the Er-added optical fiber 46. In addition, a filter 46c is inserted after the optical isolator 47 and the star coupler 48.
An optical signal transmitted from the center office 400, for example, in the 1.55 .mu.m band, is amplified by the Er-added optical fiber 46 excited by a semiconductor laser of the 1.48 .mu.m band, and then propagates to the each receiver 49 through the star coupler 48 and optical fiber 49a. Then, by using the OTDR 41 mounted with a semiconductor laser, for example, of the 1.31 .mu.m band range, it is possible to monitor the conditions of the optical fiber 44 at the center office 400.
However, in this optical communication system, since the optical isolators are inserted to the optical transmission line constituted by the optical fiber 44, the Er-added optical fiber 46 and the star coupler 48, it is impossible to transmit a optical signal from a receiver 49 to the center office 400. In addition, in the same manner, the region capable of being monitored by the OTDR 41 is limited to the section A from the center office 400 to the optical isolator 45.